The invention concerns a method for focusing of disk-shaped objects having patterned surfaces. This is done by regulated adjustment of the measurable distance from a carrier plane serving as support for the objects to a reference plane.
The method is applicable predominantly to process inspection of semiconductor wafers during the manufacturing process.
The purpose of an inspection during the manufacture of integrated circuits is to scan both unpatterned and patterned semiconductor wafers one image field at a time, to detect defects that are present, and lastly to assign them to defect groups by classification.
DE 44 10 603 C1 has disclosed a method that is the basis for the recognition of defects (specific image point features) of an image acquired of the surface of the semiconductor wafer. Direct defect detection of this kind on the basis of natural distinguishing features between background and defect and the typical properties of various types of defects can, in contrast to reference/actual comparison, largely dispense with comprehensive information about the reference pattern, since it proceeds from specific basic rules and rule deviations in the context of image recognition. Rules and rule deviations of this kind are defined substantially by geometrical and color features of the reference pattern or reference layer, and of the defect.
It is a characteristic of the method that an intermediate image generated by image point classification contains image point features as probabilities (converted into gray values) of membership in regions that have become known by training, including the gray value scales of transitions between adjacent regions.
For the analysis of image contents using the basic idea of process inspection and quality control, it is particularly interesting to know how and with what quality the patterns and defects are being imaged as three-dimensional structures with the aid of optical systems.
Since the actual appearance of patterns and defects of the surface of the semiconductor wafer depicted in the image changes as a function of the focus state, the setting of that focus state is extraordinarily important.
Known optical methods for analyzing a focus state have the disadvantage of not satisfactorily dealing with large reflectance fluctuations over smaller and larger areas of the semiconductor wafer, or with further influencing factors such as wedge errors, roughness, ripple, or fluctuations in pattern height in the image field which in some cases are greater than the depth of field. In the analysis result, for example, reflectance fluctuations simulate nonexistent height fluctuations or stage running errors that supposedly need to be controlled out.
Image point classification in the form of color classification is performed according to the method as defined in DE 44 10 603 C1. The method is based on the fact that color valences belong to undisturbed pattern components and can be estimated by sample analysis. The image point classification can be described in terms of their statistics by ellipsoidal clusters in the color space. The image points suspected of being defective yield defect clusters located outside the cluster and the separation of the defect clusters from the clusters of undisturbed patterns is considerably disrupted.
The consequence is an increase in the pseudo-defect rate and a decrease in recognition accuracy. Pseudo-defects are phenomena in the final inspection presentation that are mistakenly recognized by the inspection system as defects on the basis of their properties.
It is the object of the invention to make available to the image processor images that result in an increase in the accuracy with which defects are recognized, so that defect recognition becomes more reliable.
According to the present invention, the object is achieved by a method for focusing during the imaging of patterned surfaces of disk-shaped objects, by regulated adjustment of the measurable distance from a carrier plane serving as support for the objects to a reference plane, in that:
the distance established by regulated adjustment is corrected by an unregulated preset value to be utilized at least for a subregion of the surface.
The preset value valid for a subregion is ascertained on the basis of an image sequence, for at least one position in a substantially flat reference region of the surface, which is acquired using various preset values of the focus regulation system. The quality of the focus state is evaluated in accordance with at least one rule.
In a first embodiment, the preset value belonging to the highest-contrast image determined by image processing is used to establish a corrected distance.
In a second embodiment, the preset value belonging to the image having a minimum number of defects is used to establish a corrected distance.
In a third embodiment, the preset value belonging to the image having a minimum number of defect pixels in proportion to the total defect area. The present value is used to establish a corrected distance.
Lastly, in a fourth embodiment, the preset value belonging to the image having a minimum number of pseudo-defect pixels is used to establish a corrected distance.
The reference region is selected based on the lowest gradient of the height profile of the surface, which is to be determined by ascertaining the values of the distance from the carrier plane to the reference plane, at locations of repeating height marks, for the preset value of the focus regulation system at which at least one rule is adhered to.
During inspection in the form of a defect search one image field at a time, the preset value of the focus regulation system ascertained for the image field position is used for focusing.
For defect classification, in which an image that contains a defect to be classified is compared to a reference image, acquisition of the reference image is performed with the corrected distance from carrier plane to reference plane ascertained for the reference image position.
To acquire the image with the defects to be classified, the value of the distance from carrier plane to reference plane used for focus setting is the one belonging to the highest-contrast image of a series of images with the defect to be classified, for which the preset value of the focus regulation system ascertained for the associated image position is varied in steps.
The invention thus provides, as the basis for focus setting, at least one preset value which is determined by training on the basis of reference regions and rules directed toward the result.
The result of an adaptive control system is principally that specific properties of the object are taken into account in focusing for purposes of defect recognition and classification. In addition, instrumental influences having a disruptive effect on focusing are eliminated.